1. Field of the Invention
The present invention relates to a display method of a liquid crystal display device.
2. Description of the Related Art
In recent years, a performance of a liquid crystal display (hereinafter, called as an “LCD”) has been improved, and the LCD begins to spread to the conventional television field where a cathode ray tube (hereinafter, called as a “CRT”) is chiefly used.
The LCD uses transistors as a select switch for each pixel, and adopts a display method (hereinafter, called as a “hold-type display”), in which a displayed image is held for 1 frame period. In contrast, in CRT, a display method (hereinafter, called as an “impulse-type display”), in which a selected pixel is darkened immediately after the selection period of the pixel, is adopted. Thus, the LCD is different from CRT in time axis characteristic in an image display. Therefore, when the motion image is displayed, image deterioration such as blurring the image etc. is caused. This reason will be easily explained.
When an observer follows and observes the moving object of the motion image (when the eyeball movement of the observer is a following motion), even if the image is rewritten, for example, in 60 Hz, the eyeball has a characteristic to smoothly follow the moving object.
The black is displayed between each frame of the motion image rewritten in 60 Hz in case of the impulse-type display like the CRT. That is, the black is displayed excluding a period when the image is displayed, and 1 frame of the motion image is presented respectively to the observer as an independent image. Therefore, the image is observed as a clear motion image in the impulse-type display.
However, in the hold-type display, the displayed image of 1 frame of the motion image is held for 1 frame period, and is presented to the observer during the corresponding period as a still image. Therefore, even though the eyeball of the observer smoothly follows the moving object, the displayed image stands still for 1 frame period as shown in FIG. 1A. Therefore, the shifted image is presented according to the speed of the moving object on the retina of the observer as shown in FIG. 1B. Accordingly, since the observer perceives the image with which the shifted images are overlapped, an impression that the motion image is obscure is given to the observer. In a word, a sharpness of the motion image is lost. In addition, since the deviation between the images presented on the retina of the observer becomes large when the velocity of the motion image becomes large, the impression that the image is more obscure is given.
On the other hand, there is a white brightness as a factor to decide the picture quality of the motion image besides the factors as mentioned above.
In the CRT, the amount of the current flowing to the electron gun is controlled according to the average brightness level of the image signal of 1 frame (hereinafter, called as an “APL”). This reason is as follows. A disadvantage such that a load of a high-voltage circuit becomes too large occurs when a high-voltage current is flown to the electron gun according to the image signal in case of a high APL image (i.e., bright image on the entire screen). Therefore, the CRT comprises a circuit (hereinafter, called as an “ABL circuit”), which automatically controls brightness corresponding to the APL and a circuit (hereinafter, called as an “ACL circuit”), which automatically controls the contrast ratio.
For example, when the image signal with the high APL is displayed on the CRT, the amount of the current flowing to the electron gun is limited by an operation of the ABL circuit. Thereby, the brightness of the entire screen lowers. However, the ACL circuit operates at this time, the contrast of the image signal is increased, and a dark part is displayed more darkly. Since a relative contrast becomes high in spite of lowering the brightness of the entire screen, a high dynamic range image can be obtained with such a processing. In contrast, when the image signal with the low APL is displayed, the punched-up image with high contrast can be similarly obtained since the brightness of a bright image area becomes large.
On the other hand, in the LCD, it is preferable to reduce the impulse rate (ratio of which the image is displayed for 1 frame period), when only a priority is given to a sharpness of the motion image. However, when the impulse rate is reduced, the white brightness is insufficient. Therefore, the contrast ratio lowers due to insufficiency of the white brightness and the reality of the motion image lowers when the image with the high APL is displayed. For example, if the brightness of the backlight is raised to supplement insufficiency of the white brightness, oppositely, the entire screen becomes whitish when APL is low and the image is dark.
As described above, the picture quality of the motion image is decided by a sharpness of the displayed motion image and white brightness. However, there is a disadvantage that the image becomes obscure when the motion image is displayed, and the sharpness is lost in the conventional liquid crystal display device. To solve such a disadvantage, when reducing the ratio of the display period of the image, that is, the ratio of the black display period is enlarged, there is a disadvantage that the power of the motion image lowers because of the decrease in dynamic range due to the white brightness insufficiency.
To cancel the blurring phenomenon, the field inversion method is proposed (see Japanese Patent Application KOKAI Publication No. 2000-10076). This is a method of controlling the transmitting of the light in an analog fashion in one polarity, using the operation characteristic of the monostable liquid crystal material which does not transmit the light in the other polarity, dividing 1 frame into two fields, that is, first and second fields, transmitting the light in the first field, and not transmitting the light in the second field. A display device of the liquid crystal panel using a bent-alignment cell is proposed (see, Japanese Patent Application KOKAI Publication No. 11-109921). A display method in each proposal is close to the impulse display by providing an image display period and the black display period.
However, in the field inversion method, display duty is just only 50%, since the application time of the voltage to two poles is equal so that the DC component should not remain in the liquid crystal material. The display duty is defined by the following equation.display duty=display period/(display period+non-display period)×100  (1)
In addition, a crosstalk is occurred easily in the field inversion method.
In a method of dividing the field, it is necessary to increase the number of screen dividings to change the display duty. Therefore, an irregular display (brightness change like the tie suiting) occurs by the difference of the signal line driving circuit. Since it is necessary to change the scanning line driving frequency in order to change the display duty, it is more difficult to set the display duty in detail. Therefore, the high quality display cannot be obtained according to the display image.
There are many liquid crystal display devices in which the number of gray-scales of each color of RGB (R=red, G=green, B=blue) to express the color. However, a large number of display colors, such as eight bits, ten bits, come to be required in the future. Therefore, the number of colors is increased by using a frame rate control (hereinafter, called as an “FRC”) technology, which displays two or more times for 1 frame period. However, in inventors experiment, even if the number of colors is reduced in the motion image from the number of colors in the still image, it is partly confirmed not to be able to recognize the difference so much.
When all pixels in the display area are the same alignment (for example, a first alignment), 1 frame is divided into two fields, writing by +polarity is performed in the field of the first half and erasure by −polarity is performed in the field of the latter half to perform the exchange drive. In this case, one scanning line period is a half of conventional ones by dividing 1 frame into two fields. The writing insufficiency might be occurred, and the contrast might be lowered.